It is known to use endocardiac leads, also called catheters, for stimulating a heart and for sensing electrical signals therefrom.
An endocardiac lead presents two ends. One end is called the proximal end and is connected within the connector head of a pacemaker to a source of electrical energy. The other end is called the distal end and is typically brought into contact with the internal wall of the heart muscle. The distal end includes a conductive electrode that is connected to an electrically conductive part, such as a coiled conductive wire, encased in a flexible sheath. The sheath may be made, for instance, of silicone rubber.
Endocardiac leads are typically introduced within the heart through a vein, for example, the cephalic vein. When the lead has to be placed inside the atrium, the walls of which are relatively smooth, it is advantageous to provide an active fastening means in the distal end to secure the distal end to the tissue. One known fastening means is a helix shape that is, for example, inserted into the cardiac tissue through rotation.
It is known to provide an endocardiac lead with a helix that is secured to, and protrudes from, the distal end. In this device, when the distal end of the lead has been brought by the surgeon to the desired location inside the atrium, lead is rotated so that the helix penetrates the tissue to fix the distal end in position. One problem with this device is that the protruding helix may, during insertion or rotation of the lead, cause wounds in the vein, or catch on the cardiac valves, complicating the procedure.
It has also been suggested to provide on the distal end a retractable helix. Such a helix is placed in a retracted position within the distal end while the lead is being inserted. The lead is provided with means for causing the helix to rotate and protrude out into tissue, after the lead has been suitably placed.
U.S. Pat. No. 4,217,913 refers to placing the helix within the distal end of the lead and connecting it to a cylindrical actuating end piece that is accommodated within the lead. The end piece is provided with a slit for cooperating with a stylet introduced into the lead for rotating the helix through the intermediary of the end piece. When pulling the lead out, the helix rests against a monofilament which extends radially across the lead while its ends are thermowelded to the external surface of the lead. A drawback of this construction is that the diameter of the lead at the distal end is large compared to the proximal portions of the lead and to leads not having a retractable helix. Another drawback is that it is complicated to manufacture.
The known leads comprising a retractable helix fastening means have, at least at their distal end an undesirably and comparatively large diameter. This is a not an insignificant drawback during an invasive procedure such as the insertion of the lead through a vein. It also is a problem in connection with rotatably driving the helix fastening means. This is because constrictions which are locally compressing the lead in a radial direction are also hindering the rotation of the means for actuating the fastening means.
There is thus a continuing need for improved endocardiac leads that can be secured to tissue, more particularly for such leads that are easier to insert and install in the desired tissue location. There is also a need for an improved fastening means for tightly securing tissue contacting portions of medical devices, catheters and probes to selected tissue sites.